1. Field of the Invention
The invention relates to an image heating apparatus like a heating and fixing device mounted on an image forming apparatus such as a copier or a printer, and particularly to an image heating apparatus of the induction heating type.
2. Related Background Art
A copier of the electrophotographic type or the like is provided with a heating apparatus for fixing a toner image transferred onto a sheet such as recording paper or a transferring material which is a recording medium on the sheet.
The heating apparatus has, for example, a fixing roller called also a heat roller for thermally fusing the toner on the sheet, and a pressure roller brought into pressure contact with the fixing roller to nip the sheet between the pressure roller and the fixing roller. The fixing roller is formed into a hollow shape, and a heat generating member is held on the axis of the fixing roller by holding means. The heat generating member is comprised of a tubular heat generating heater such as a halogen lamp, and generates heat by a predetermined voltage being applied thereto. Since the halogen lamp is located on the axis of the fixing roller, the heat generated by the halogen lamp is uniformly radiated to the inner wall of the fixing roller, and the temperature distribution in the outer wall of the fixing roller becomes uniform in the circumferential direction thereof. The outer wall of the fixing roller is heated until the temperature thereof becomes a temperature suited for fixing (e.g. 150 to 200xc2x0 C.). In this state, the fixing roller and the pressure roller are rotated in opposite directions while being in pressure contact with each other, and nip and convey the sheet to which the toner adheres therebetween. In the pressure contact portion (hereinafter referred to also as the nip portion) between the fixing roller and the pressure roller, the toner on the sheet is fused by the heat of the fixing roller, and is fixed on the sheet by the pressure acting from the two rollers.
However, in the above-described heating apparatus provided with the heat generating member comprised of a halogen lamp or the like, the radiant heat from the halogen lamp is utilized to heat the fixing roller and therefore, a relatively long time has been required as the time until the temperature of the fixing roller reaches a predetermined time suited for fixing after a power source has been turned on (hereinafter referred to as the warm-up time). This has led to the problem that in the meantime a user cannot use the copier and is compelled to wait for a relatively long time.
On the other hand, if a great deal of electric power is applied to the fixing roller to attempt the shortening of the warm-up time and improve the operability for the user, there has arisen the problem that the consumed electric power in the heating apparatus is increased against energy saving.
Therefore, to enhance the commercial value of the copier or the like, more attention and importance have been attached to contrive the compatibility of the energy saving (lower consumption of electric power) of the heating apparatus and the operability (quick print) for the user.
As an apparatus which meets such a requirement, there has been proposed a heating apparatus of the induction heating type utilizing high frequency induction as a heat source, as shown in Japanese Patent Application Laid-Open No. 59-33787.
The induction heating apparatus is such that a coil is concentrically disposed in a hollow fixing roller comprising a metallic conductor, and by a high frequency magnetic field generated by a high frequency current being made to flow to the coil, an induction eddy current is created in the fixing roller, and the fixing roller itself is caused to generate Joule heat by the skin resistance of the fixing roller itself.
According to the heating apparatus of the induction heating type, electro-thermal conversion efficiency is very much improved and therefore, the shortening of the warm-up time becomes possible.
In such a heating apparatus of the induction heating type, however, a great current of several A to several tens of A flows through the coil, and this has led to the problem of the temperature rise by the Joule heat generation of the coil itself.
Also, when an induction coil is disposed in the internal space of a heating member, efficient heat radiation does not take place and the temperature rise of the coil becomes very great.
When the temperature rise of such an induction coil occurs, there has been the problem that for example, the covering of the induction coil is fused by heat and the insulativeness is spoiled.
So, as disclosed, for example, in Japanese Patent Application Laid-Open No. 54-39645 and Japanese Patent Application Laid-Open No. 10-282826, there has been made a proposition to provide a cooling mechanism such as blowing means to suppress the temperature rise of the induction coil.
However, the provision of a cooling mechanism such as blowing means leads not only to a correspondingly higher cost, but also to the necessity of securing a space therefor. Further, it has led to the waste of energy to indirectly cool the heat generated by consuming electric power.
The present invention has been made in view of the above-noted problems and an object thereof is to provide an image heating apparatus which can suppress the temperature rise of an induction coil.
Another object of the present invention is to provide an image heating apparatus which suffers little from the loss of a power supply.
Still another object of the present invention is to provide an image heating apparatus which is excellent in heat generating efficiency.
Yet still another object of the present invention is to provide an image heating apparatus comprising:
a heating member; and
an excitation coil for generating a magnetic field to induce an eddy current in the heating member,
wherein of the total electric power applied to the coil, the relation between active power W and reactive power Wxe2x80x2 is
0.1xe2x89xa6W/(W+Wxe2x80x2)xe2x89xa60.8. 
Further objects of the present invention will become apparent from the following detailed description when read with reference to the accompanying drawings.